Biodiesel fuels have drawn much attention because they make low emission of carbon dioxide when used, and they can be used as alternate fuels for fossil fuels. Biodiesel fuels can be obtained by ester exchange reaction of vegetable oils, and it has been known that this reaction is accompanied by formation of glycerin as a by-product. Along with an increase in the demand and production of biodiesel fuels year by year, the formation of glycerin as a by-product has also been increased. In such a social climate, it has been required to make effective use of glycerin.
Acrolein is a compound obtained by intramolecular dehydration of glycerin and is useful as a raw material for acrolein derivatives such as 1,3-propanediol, methionine, acrylic acid, 3-methylthiopropionaldehyde, and water-absorbent resins. The production of acrolein from glycerin is to widen the application range of glycerin to the use as a raw material for acrolein derivatives. Therefore, it makes possible effective use of glycerin and also widens the application range of the by-product in the production of biodiesel fuels, thereby improving the economic value of the production of biodiesel fuels.
It has heretofore been known in the art that glycerin is used as a raw material for acrolein. For example, Japanese Patent Laid-open Publication (Kokai). No. Hei 6-192147 discloses that acrolein is obtained by dehydration of glycerin in the process of obtaining 1,2-propanediol and 1,3-propanediol from glycerin. Japanese Patent Laid-open Publication (Kokai) No. 2005-213225 discloses that acrolein is obtained by gas-phase dehydration of glycerin in the process of obtaining acrylic acid from glycerin.
Acrolein is usually produced using a catalyst for promoting intramolecular dehydration of glycerin. International Publication WO2007/119528 corresponding to the International Patent Application by the present applicant discloses that yttrium salt crystals, lanthanum salt crystals, cerium salt crystals, or samarium salt crystals of phosphoric acid are used as a catalyst. In the process for producing acrolein using these salt crystals, when glycerin gas is employed as a raw material, the deposition of carbonaceous substance on the surface of a catalyst, which is one of the factors for the deactivation of the catalyst, is suppressed. Similarly to how the suppression of such a deposition is being desired in the production of acrolein, a process for producing acrolein in high yield is also inevitably desired.